forkproxy: handle mnts for abstract unix sockets
forkproxy: handle mnts for abstract unix sockets

We only need to attach to the mount namespace for non-abstract unix
sockets.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

hallyn/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,mjeanson/lxd,stgraber/lxd,Skarlso/lxd,hallyn/lxd,Skarlso/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,stgraber/lxd,Skarlso/lxd,Skarlso/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,mjeanson/lxd,atxwebs/lxd,lxc/lxd,hallyn/lxd,mjeanson/lxd,hallyn/lxd,hallyn/lxd,atxwebs/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd